Vacuum vapor coating with metals of high vapor pressure

ABSTRACT

The substrate is vacuum vapor coated with a metal having a high vapor pressure at its melting point so that it tends to sublime by evaporating the coating metal from a binary alloy of the coating metal and a second metal, the alloy having a melting point lower than that of the coating metal, and the second metal having a vapor pressure at the alloy melting point much lower than that of the coating metal.

United States Patent Gimigliano 1 Jan. 25, 11972 [54] VACUUM VAPOR COATING WITH METALS OF HIGH VAPOR PRESSURE [21] Appl. No.: 853,525

Related US. Application Data [63] Continuation-impart of Ser. No. 663,953, Aug. 29,

1967, abandoned.

[52] U.S.Cl ..1l7/107,ll7/l07.1, 117/131 [51] Int. Cl ..C23c 13/02 [58] Field ofSearch ..117/107, 107.1,131

[56] References Cited UNITED STATES PATENTS 2,432,657 12/1947 Colbert et al. ....1 17/107 X 2,882,377 4/1959 Rinehart ...1 l7/211 2,900,282 8/1959 Rubens ..1 17/107 X 2,962,393 11/1960 Ruckelshaus 17/107 X 3,472,691 10/1969 Kooy et al ..l 17/107 X OTHER PUBLICATIONS Powell et al., Vapor Deposition, 1966, Wiley & Sons, lnc., pp 233- 235 and 242- 246.

Dushman et al., Scientific Foundation of Vacuum Technique, Wiley & Sons, lnc., 1962, pp. 710- 722.

Primary ExaminerAlfred L. Leavitt Assistant ExaminerWm. E. Hall Attorney-G. R. Harris and T. A. Zalenski [5 7] ABSTRACT The substrate is vacuum vapor coated with a metal having a high vapor pressure at its melting point so that it tends to sublime by evaporating the coating metal from a binary alloy of the coating metal and a second metal, the alloy having a melting point lower than that of the coating metal, and the second metal having a vapor pressure at the alloy melting point much lower than that of the coating metal.

10 Claims, No Drawings VACUUM VAPOR COATING WITII METALS OF HIGH VAPOR PRESSURE This application is a continuation-in-part of application Ser. No. 663,953, filed Aug. 29, 1967, now abandoned.

In the vacuum vapor coating of a substrate with a metal, it is necessary to heat the coating metal to its vaporization temperature. It is conventional to do this by melting the coating metal in a crucible or otherwise and controlling the temperature of the molten coating metal so that it vaporizes at the desired rate. Certain metals, to be discussed more fully below are not readily handled in this way. These are the metals, which, under the conditions obtaining in vacuum vapor coating, have high vapor pressures at their melting points. The vacuum under which metals are evaporated is a pressure of the order of millimeters of mercury. By high vapor pressure" I mean vapor pressures more than about a hundred times the pressure in the vacuum vapor coating chamber. Metals with such high vapor pressures tend to sublime, i.e., to pass directly from the solid to the vapor state. It is much more difficult to control sublimation than it is vaporization of a metal from the liquid state. Furthermore, it is much more difficult to feed a solid metal to a subliming zone for continuous operation than it is a liquid or a solid metal to a liquid bath.

It is an object of my invention to provide a process of vaporizing high vapor pressure coating metals from the liquid state. It is another object to provide a process of vaporizing such a metal from a molten binary alloy having a melting point below that of the coating metal alone. It is another object to provide a process adapted for continuous coating of the substrate in strip form. It is still another object to provide such a process which deposits a coating containing only an insignificant amount of the alloying constituent. Other objects of my invention will appear in the description thereof which follows:

I have found that a coating metal which tends to sublime can be evaporated without difficulty from a binary alloy of the coating metal and a second metal, the proportions of the two metals being chosen so that the alloy has a melting point lower than that of the coating metal. I have found that if the second metal has a vapor pressure at the melting temperature of the alloy lower than the vapor pressure of the coating metal by a factor of I00 or more, insignificant amounts of it are found in the condensed coating metal.

My invention is particularly applicable to the vacuum vapor coating of steel with a coating metal which is resistant to atmospheric corrosion. The specific high vapor pressure coating metals for which my process is suitable are magnesium, calcium, manganese, zinc, tellurium, and chromium. The following examples illustrate my process employing these coating metals.

EXAMPLE 1 Magnesium is a coating metal with corrosion resistance properties comparable to zinc. However, at its melting point of about l,200 F., it has a high vapor pressure, about 2 millimeters of mercury, and sublimes and/or evaporates violently so that it is very difficult to feed and control in a commercial process. I have found that magnesium can be evaporated with no difficulty from an alloy of magnesium and aluminum which melts at a temperature of 900 F. or less, and that the vapor pressure of the aluminum at that temperature is so low that aluminum appears in the coating only in very small amounts, less than 0.2 percent. The aluminum content of the alloy ranges from about 25 percent to about 70 percent. The minimum melting point of the alloy, about 860 F., occurs at about 38 percent aluminum content.

EXAMPLE 2 Calcium has properties similar to those of magnesium, and a vapor pressure at its melting point (about l,650 F.) of about 2 millimeters of mercury. Calcium is readily vaporized from a calcium-aluminum alloy containing about 25 percent aluminum, which melts at l,l00 F. The vapor pressureof the aluminum at that temperature is low enough that only small amounts of it appear in the calcium metal coating.

EXAMPLE 3 Calcium is readily vaporized from an alloy of calcium and silver containing about 40 percent silver, which has a melting point of about 950 F. No substantial amount of silver appears in the calcium metal coating.

EXAMPLE 4 Calcium is readily vaporized from an alloy of calcium and copper containing about 60 percent copper, which has a melting point of about l,l00 F. Only small amounts of copper are found in the calcium metal coating.

EXAMPLE 5 Manganese melts at about 2,273 F. and has a vapor pressure at its melting point of about I millimeter of mercury. It is readily vaporized from a manganese-nickel alloy containing about 40 percent nickel, which melts at about l,870 F. Small quantities of nickel are found in the manganese coating.

EXAMPLE 6 Zinc has a vapor pressure of about 0.1 millimeter of mercury at its melting point of 790 F., and can be evaporated with a greater degree of control from a zinc-tin alloy. The melting point of this alloy containing percent tin is about 380 F. and increases with increasing amounts of zinc. The vapor pressure of the tin is low enough that it does not appear in any substantial quantity in the zinc coating.

EXAMPLE 7 Tellurium has a melting point of about 840 F. and a vapor pressure of about 0.1 millimeter of mercury at that temperature. It is vaporized without difficulty from an alloy of tellurium and tin containing 15 percent tin. The melting point of that alloy is about 770 F. The vapor pressure of the tin is low enough that tin is not found inthe tellurium coating metal in any substantial quantity.

EXAMPLE 8 Chromium is a refractory element which melts at about 3,430 F. At that temperature its vapor pressure is about 8 millimeters of mercury. It is vaporized readily from an alloy of chromium and titanium containing about 50 percent chromium. That alloy melts at about 2,550 F. Only small amounts of titanium are found in the chromium coating.

The accompanying table lists the vapor pressures of the coating metals and second or alloying metals of the examples above set out at the melting points of their alloys there mentioned, and the ratios of those vapor pressures.

I prefer to practice my process by providing in a chamber evacuated to a pressure of 10" millimeters of mercury or less a molten bath or pool of coating metal alloy as indicated in the examples. This bath is maintained molten in a crucible which is heated, preferably electrically, and maintained at the desired temperature for evaporation of coating metal. This temperature is above the melting temperature of the alloy used. An advantage of my process is that high coating rates may be realized by heating the alloy to temperatures above that at which the coating metal would sublime under the reduced pressure in the coating chamber if it were heated unalloyed.

The substrate in the form of strip is passed through the evacuation chamber over the crucible and the coating metal vapor condenses on the substrate. It is necessary for continuous operation to make additions of coating metal and second metal sufficient to maintain the coating metal bath at its required composition and these additions are made periodically by adding metal in solid or liquid form, either separately or as an alloy, or continuously by adding metal in liquid form.

While it is desirable for atmospheric corrosion protection to provide a substrate with a coating of relatively pure coating metal, the pressure of small amounts of alloy metal can be tolerated. For many years large tonnages have been sold of galvanized steel sheets coated with zinc which contains small amounts of lead on the order of 1 percent or to which small amounts of aluminum have been intentionally added on the order of 0.15 percent. in the articles provided by my process the alloying elements are present in no greater amounts.

It will be understood that the surface of the steel substrate requires preparation appropriate to the coating metal concerned. Techniques of surface preparation for steel and other substrate materials are known to those skilled in the art of vacuum vapor coating and do not form a part of my invention. Those skilled in the art also know that preheating or postheating of the substrate are required under certain conditions, and these steps constitute no part of my invention.

I claim:

1. In the process of vacuum vapor coating a substrate with a coating metal which sublimes when heated to its vaporization temperature under vacuum vapor coating conditions, selected from the group consisting of magnesium, calcium, manganese, zinc, tellurium and chromium, the improvement comprising maintaining under vacuum in a crucible a bath of a molten alloy of the coating metal with a second metal selected to form an alloy having a melting point lower than that of the coating metal and to have at the melting point of the alloy a vapor pressure lower than that of the coating metal by a factor of or more, heating the alloy to a temperature above its melting point to evaporate the coating metal therefrom, and condensing the coating metal upon the substrate, whereby the coating metal may be evaporated without subliming.

2. The process of claim 1 in which the alloy is heated to a temperature above that at which the unalloyed coating metal would sublime under vacuum vapor coating conditions.

3. The process of claim 1 in which the coating metal is magnesium, and the second metal is aluminum in amounts between about 25 percent and about 70 percent.

4. The process of claim 1 in which the coating metal is chromium, and the second metal is titanium in the amount of about 50 percent.

5. The process of claim 1 in which the coating metal is calcium, and the second metal is aluminum in the amount of about 25 percent.

6. The process of claim 1 in which the coating metal is calcium, and the second metal is silver in the amount of about 40 percent.

7. The process of claim 1 in which the coating metal is calcium, and the second metal is copper in the amount of about 60 percent.

8. The process of claim 1 in which the coating metal is manganese, and the second metal is nickel in the amount of about 40 percent.

9. The process of claim 1 in which the coating metal is zinc, and the second metal is tin.

10. The process of claim 1 in which the coating metal is tellurium and the second metal is tin in the amount of about 15 percent. 

2. The process of claim 1 in which the alloy is heated to a temperature above that at which the unalloyed coating metal would sublime under vacuum vapor coating conditions.
 3. The process of claim 1 in which the coating metal is magnesium, and the second metal is aluminum in amounts between about 25 percent and about 70 percent.
 4. The process of claim 1 in which the coating metal is chromium, and the second metal is titanium in the amount of about 50 percent.
 5. The process of claim 1 in which the coating metal is calcium, and the second metal is aluminum in the amount of about 25 percent.
 6. The process of claim 1 in which the coating metal is calcium, and the second metal is silver in the amount of about 40 percent.
 7. The process of claim 1 in which the coating metal is calcium, and the second metal is copper in the amount of about 60 percent.
 8. The process of claim 1 in which the coating metal is manganese, and the second metal is nickel in the amount of about 40 percent.
 9. The process of claim 1 in which the coating metal is zinc, and the second metal is tin.
 10. The process of claim 1 in which the coating metal is tellurium and the second metal is tin in the amount of about 15 percent. 